Fundamental parameters of cortical and cerebellar development will be Studied using retrovirus-mediated gene transfer. Through introduction of a histochemical marker gene carried by a retrovirus vector, cells will be tagged with an innocuous and stable genetic marker, enabling an analysis of fate within the developing brain of rats and mice. Patterns of proliferation, migration and lineage will be examined. The role of lineage in the determination of cytoarchitectonic boundaries will also be investigated. The protocols employed in this proposal will build upon the previous methodology developed in the prior grant period. Improvements to the existing technology will be made by way of the development of novel histochemical marker viruses.